It’s hard to believe, but it’s been 25 years since the Delicatessen Orgasm Heard ’Round the World first made it onto the movie screen. We speak, natch, of “When Harry Met Sally . . .,” Nora Ephron’s 1989 instant classic comedy, starring a youthful Billy Crystal and a pre-Botoxed Meg Ryan (trivia note: Albert Brooks and Molly Ringwald were among the original choices for the leads).

It follows a couple who meet after college, become friends and take about a dozen years to realize they’re in love. Along the way, there’s that memorable stop in Katz’s Deli, birthplace of the immortal “I’ll have what she’s having” line — uttered by director Rob Reiner’s mother, Estelle.

Timing is everything.

When Harry Burns and Sally Albright first meet — for what feels to them the world’s longest car ride — they don’t even like each other. (Actually, Sally says later, “I hated you.”)

At their next encounter, Harry barely remembers her. The third time they meet, they become friends. And they stay that way for a very long time, though now and then — but never quite simultaneously — one of them wants more. “I’ve been doing a lot of thinking,” Harry finally says, “and the thing is, I love you.” To which Sally responds, “How do you expect me to respond to this?” Never a good sign.

Which brings us to lesson No. 2:

The “sex part” doesn’t have to ruin a perfectly good friendship.

“No man can be friends with a woman that he finds attractive,” Harry declares. “He just wants to have sex with her.” Once sexual attraction does rear its head, he says, the friendship is “ultimately doomed.”

Sally, the pragmatic one, decides she’ll stick with friendship: After all, Harry’s the only person she knows in New York.

Wanting the same things — or at least thinking you do — just isn’t enough.

Photo: Columbia Pictures/Everett Collection

Sally learned that from her boyfriend Joe: “We wanted to live together, but we didn’t want to get married because every time anyone we knew got married, it ruined their relationship,” she tells Harry. “We’d say we were so lucky we have this wonderful relationship, we can have sex on the kitchen floor and not worry about the kids walking in.”

Turns out, the kitchen floor wasn’t very comfortable.

Never mind what “Love Story” told you — it doesn’t hurt to say you’re sorry. Especially if you mean it.

When Sally finally does break up with Joe, Harry goads her about seeing other people.

“I see people,” she replies. “See people?” he echoes. Angry and cornered, she lashes out — and Harry realizes he isn’t helping her at all. “Can I say something?” he offers. “I’m sorry. I’m sorry.”

For the moment, that heartfelt apology is her only consolation.

You can have everything else you want “on the side” — but when it comes to love, stick to the menu.

A friend assures Harry that marriages don’t break up because of infidelity — “It’s just a symptom that something else is wrong.” Replies Harry, “Well, that ‘symptom’ is f - - - king my wife.”

The moral: Have all the sauces you want on the side, but when it comes to love, be faithful to your entrée. With any luck, you’ll continue to have what Sally’s having.